1. Field of the Invention
The present invention relates to an aluminum nitride circuit board and a method of producing same, and more particularly, to an aluminum nitride circuit board used in a semiconductor device assembly, etc., and a method of producing same.
2. Description of the Related Art
Recently the increasingly high speed operation and integration, etc. of semiconductor devices has led to a demand for an upgrading of the properties used in these devices, circuit boards and the semiconductor package, etc.
For example, a large amount of heat is generated by a logic circuit element and thus the circuit board must have a high thermal conductivity. Further to lower thermal stress of such an element, a circuit board having a thermal coefficient of expansion very close to that of a semiconductor device is required. Furthermore, to achieve a high propagation speed, a wiring length must be shortened as much as possible, a material having a low electric resistance must be used for the circuit pattern, and an insulating material having a low dielectric constant also must be incorporated.
Among ceramic materials used for circuit boards or as a package material, the most widely-used is an alumina ceramic. An alumina ceramic has a low thermal conductivity of 20 W/mK and a thermal coefficient of expansion of about 7.times.10.sup.-6 /.degree.C., which is about twice that of silicon. Further, the dielectric constant of an alumina ceramic is 9 or more, and since its sintering temperature is very high, only a high melting metal such as tungsten or molybdenum can be used as the circuit pattern material in a co-firing process.
A metal having such a high melting point, however, has a high electric resistance, and thus the alumina ceramic is not suitable for this purpose.
Therefore, to replace the alumina ceramic, ceramics such as an aluminum nitride ceramic, a silicon carbide ceramic, a mullite ceramic, and a low temperature firing ceramic have been developed and are now in use.
Among these ceramic materials, the silicon carbide ceramic has a thermal coefficient of expansion of 3.7.times.10.sup.-6 /.degree.C., which is near to that of silicon, and has a high thermal conductivity. Particularly, the silicon carbide ceramic using a beryllia as an additive has a thermal conductivity of about 270 W/mK. Nevertheless, it is difficult to form a multi-layer circuit using the silicon carbide ceramic, and further, the silicon carbide ceramic has a remarkably high dielectric constant of about 40.
The mullite ceramic has the advantages of a thermal coefficient of 4.5.times.10.sup.-6 /.degree.C., which is close to that of silicon, and a small dielectric constant of 7.3, but has the disadvantages of a low mechanical strength and the necessity to use a high electric resistance material such as tungsten or molybdenum as a circuit pattern material, as required by the alumina ceramic, and further, has a thermal conductivity lower than that of the alumina ceramic.
The low temperature firing ceramic has the advantages of possessing a thermal coefficient of expansion and a dielectric constant superior to that of the alumina ceramic, and materials having a low electric resistance such as gold, silver, and copper, etc. can be used as a circuit pattern material. Nevertheless, the low temperature firing ceramic has the disadvantages of an inferior mechanical strength, a low highest use temperature, and the lowest thermal conductivity among the above-mentioned materials.
As explained above, each ceramic material has advantages and disadvantages, and accordingly, an aluminum nitride ceramic will now be explained by comparison with the above ceramic materials. The aluminum nitride ceramic has a thermal coefficient of expansion of 4.5.times.10.sup.-6 /.degree.C., a thermal conductivity of 170 to 260 W/mK, a dielectric constant smaller than that of alumina ceramic, and a high mechanical strength. Thus, the aluminum nitride ceramic has the must balanced properties of the various ceramic materials. Nevertheless, since the aluminum nitride ceramic has a high firing temperature of 1700.degree. C. or more, a high melting point metal must be used as the circuit pattern material in a co-firing process, for example, tungsten, which has a melting point of 3382.degree. C. The melting points of gold, copper, and silver are, respectively, 1063.degree. C., 1083.degree. C., and 960.5.degree. C., which are quite low compared with that of tungsten. Since gold, copper and silver are melted and evaporated at the usual firing temperature of an aluminum nitride ceramic they can not be used as the circuit pattern material in the co-firing thereof.
The electric resistance of a circuit pattern material can slow the signal transmission speed and cause a power loss, and the larger the dielectric constant the longer the delay in the propagation of signals when high frequency signals are transmitted in a circuit. Further, in a rectangular wave which transmits on-off signals the wave shape transmitted is deformed by the electrostatic capacity stored near the circuit and the electric resistance of the circuit material itself. The on-off signal is formed by a time of reaching a constant voltage of a rectangular wave, i.e., a threshold voltage, and when the wave shape is deformed, the time of reaching the threshold voltage is delayed, and therefore, the signal transmission speed is lowered. To prevent this deformation of the transmission wave shape, it is necessary to lower the electrostatic capacity near the circuit pattern, (in other words, the dielectric constant near the circuit pattern) and to make the value of electric resistance of the circuit pattern material low. The signal delays in each circuit pattern are a serious problem, since a large number of computing operations are carried out at a high speed in a logic circuit.
Since a conventional aluminum nitride circuit board or package has a high electric resistance of circuit pattern material, e.g., tungsten etc. in spite of superior properties of the ceramic board itself, the high speed transmission of signals in a device cannot be made higher than in a conventional alumina ceramic package.